Initial Agehardening in Al-Zn Alloys

Abstract

Measurements of the electrical resistivity, simple tensile tests and stress relaxation tests are made to investigate the hardening mechanism at the early stage of ageing in Al-Zn alloys. There exists a good linear relationship between the flow stress σ and the square root of the plastic strain ε^1⁄2 at any stage of ageing for pure Al and Al-Zn alloys deformed at \dotε=10⁻⁴ s⁻¹ and 77 K. The strain independent flow stress σ₀ obtained by extrapolating ε^1⁄2 to zero increases drastically in the concentrated alloys as ageing proceeds. This initial agehardening can be explained by the interaction between dislocations and the coherent strain fields as proposed by Gerold et al.
The specific activation volume ΔV^∗ concerning the thermally activated shearing of GP zones by dislocations is obtained from stress relaxation tests, assuming that the rate controlling process is a consecutive one of the dislocation intersection and the shearing of GP zones by dislocations for Al-Zn alloys. As ageing proceeds, ΔV^∗ increases until the electrical resistivity reaches its maximum. Marked changes in ΔV^∗ may results from a considerable increase in the number of small GP zones at the initial early stage of ageing.